The present invention relates to an apparatus for treating the ends of a large-diameter welded pipe and, more particularly, to an automated apparatus for continuously cutting tab plate of a large-diameter straight seam steel pipe after welding and removing inside beads of the ends of the pipe. The term "treating the ends of a pipe" or "pipe-end treatment" used herein and in the claims is to be understood to mean such operations as cutting tab plates and removing inside beads from the ends of the pipe.
In conventional pipe-end treatment of this kind, the pipe is treated either with the seam upward or with the seam downward. The pipe-end treating apparatus according to the present invention is applied to pipe-end treatment of the type wherein the seam is upward.
In order to improve the quality of products over the entire length of the seam in the manufacture of straight-seam pipes, it is conventional, as shown in FIG. 2, to connect tab plates to an end face of a formed pipe P along a seam line B thereof and to perform preparatory welding over a distance of 10-20 cm on the tab plates T. In welding along the straight seam B, however, welding defects usually occur at the beginning and at the end of the welding seam. Therefore, tab plates are attached to the ends of the pipe before forming so that these welding defects are confined to the tab plates.
After welding the pipe, the tab plates become unnecessary and must be immediately removed. Heretofore, this tab removing work has been a task for manual labor and thus has been low in efficiency. Furthermore, being a simple and repetitive operation, it has been considered undesirable in terms of work strategy.
In view of the situation described above, mechanization is most eagerly desired for the step of cutting off the tab plates and removing the inside beads in the neighborhood of the ends of the pipe. However, mechanization of this step is very difficult. Some known automated apparatuses for carrying out work of this kind are those described, for example, in Japanese Patent Public Disclosure No. 181530/83 Official Gazette and Japanese Utility Model Public Disclosure No. 171056/84 Official Gazette. However, these known apparatuses have not been successful in satisfying the requirements described below.
That is, in mechanizing the pipe-end treating operation, the following problems must be fully overcome:
(i) In the ends of a straight seam pipe after welding, the tab plates usually fail to match each other along the seam line B as shown in FIGS. 2 and 3. In such a case, it is desired that the mismatched pipe end is cut as little as possible, as shown by a dashed line C in FIG. 3, from the viewpoint of the need to maintain yield. It is also desirable for the pipe end to be cut at a small angle at the start and end points of cutting in order to minimize adverse effects on the succeeding steps (such as expanding and bevelling steps).
(ii) Since gas cutting is used, when cutting oxygen is blown at a cutting start point A, a defect is caused in the pipe end, as shown in FIG. 3, which deteriorates the sectional shape and lowers the yield of the pipe.
(iii) The thickness of the bead portion becomes 1.2-2 times that of the pipe itself and this increased thickness sometimes makes it impossible to cut the bead portion in the same cutting condition as that of the pipe itself.
(iv) In order to continue a stable cutting operation even when the pipe diameter changes or when the pipe curvature is not uniform in the circumferential direction, it is necessary to keep the cutting torch being used at an adequate distance from the outer peripheral surface of the pipe and in a position substantially at right angles to a tangent of the cutting point during the cutting operation.
(v) Mechanization will not be beneficial if the construction employed is too complicated or accuracy is low.
When reviewed from the view-point of the requirements mentioned above, the known apparatuses referred to have certain disadvantages. That is, the apparatus described in Japanese Patent Public Disclosure No. 181530/83 Official Gazette requires electrical calculation in order to detect the degree of mismatch of the pipe end along the seam line, thereby requiring a complicated detecting device and increasing the cost thereof. Further no effective measure is available for avoiding the defect at the cutting start point A, thereby resulting in a reduced yield rate. Since the cutting at the pipe end is performed at a given low speed so as to effectively cut the bead portion, the efficiency is low. Furthermore, in this known apparatus which is intended to treat pipes with the seam downward, since a tab plate cutting device and an inside bead removing device interfere with each other on the same side, this apparatus requires a completely different concept in design from the type of apparatus which is intended to treat pipes with the seam upward.
On the other hand, while the apparatus described in Japanese Utility Model Public Disclosure No. 171056/84 is intended to treat pipes with the seam upward, it is not provided with means for detecting mismatch at the pipe end and it cuts the tab plates along a uniform locus, resulting in a serious loss of accuracy and reduction in yield rate. Furthermore, this apparatus is not provided with any effective means of avoiding the defect at the cutting start point A or with any effective measures with respect to cutting the bead portion of the pipe.
In a large-diameter straight seam welded pipe, the inside weld bead represents an obstacle in clamping during circumferential welding of the pipe and must be completely removed at least from the ends of the pipe. This inside bead removing operation is deemed undesirable from the viewpoint of efficiency and job strategy because it is a simple and repetitive manual operation and it has long been desired that the working environment be improved because a large quantity of dust is caused when removing the inside bead by a grinding operation.
Accordingly, several apparatuses have been suggested in the past for removing the inside bead, such as those described in said Japanese Patent Public Disclosure No. 181530/83 Official Gazette and Japanese Utility Model Public Disclosure No. 171056/84 Official Gazette. However, since the straight seam welded pipes are not perfectly circular in section (see FIG. 6) and rather deficient in straightness (see FIG. 7), none of these known apparatuses has been completely successful in dealing with such pipes for the reasons explained below.
In the apparatus described in Japanese Patent Public Disclosure No. 181530/83 Official Gazette, as shown in FIG. 4, the inside weld beads of the welded pipe P are removed by a grinding wheel G which rotates while swinging peripherally around the pipe with its rotary shaft maintained parallel to the line of the inside weld beads. On the other hand, in the apparatus described in Japanese Utility Model Public Disclosure No. 171056/84 Official Gazette, as shown in FIG. 5, the inside weld beads are removed by the grinding wheel G with its rotary shaft maintained perpendicular to the line of the weld beads.
However, the welded pipe P is, as shown in FIGS. 6 and 7, neither perfectly round nor perfectly straight. Particularly at the ends from which the beads are to be removed, the pipe generally has a chestnut-shaped cross section and an convex longitudinal section on the side of the weld beads B. The deviation from perfect roundness and perfect straightness varies from pipe to pipe and is particularly remarkable in the stage prior to a pipe expanding step (or bead removing step), which is mainly undertaken for the purpose of maximizing the roundness and straightness of the pipe.
In the case where the apparatus shown in FIG. 4, or FIG. 5 is applied to the welded pipe P as described above, when the welded pipe P has a markedly chestnut-shaped cross section, the apparatus shown in FIG. 4 in particular has a disadvantage in that the base metal of the pipe on both sides of the weld beads B is ground away. This disadvantage becomes greater as the diameter of the grinding wheel G becomes larger.
In the welded pipe P, the lower limit of thickness is defined, and strict control of thickness is required for the beads and the steel in their neighborhood to avoid any concentration of stress. The grinding wheel G is necessarily complicated in regard to the fact that the position of the grinding wheel G relative to the weld beads B must be changed every movement in response to abrasion caused in the grinding wheel G. The grinding wheel G involves a further problem in that dust and vibration caused by grinding decrease the mechanical accuracy of machines disposed in the neighborhood of it. The grinding wheel G must have a relatively large diameter and grain size to ensure grinding efficiency. On the other hand, however, the large diameter and grain size of the grinding wheel G results in a reduction in the grinding accuracy and the possibility of the base metal of the pipe being ground away as described above.
The apparatus shown in FIG. 5 requires the use of a grinding wheel G having a small diameter to avoid the possibility of grinding the base metal away. The grinding wheel G with a small diameter has certain disadvantages, however, such as low grinding efficiency and so on.
Accordingly, an object of the present invention is to provide an apparatus for treating the ends of large-diameter welded pipe, in which all of the problems of the prior art described above can be obviated.
Another object of the present invention is to provide an apparatus for treating the ends of large-diameter welded pipe which is simple in structure, economical to manufacture, and offering high and stable work accuracy.